Arrange a rack in the middle of the oven and heat the oven to 350°F. Line an 8x8-inch baking pan, preferably metal, with parchment paper, leaving an overhang on two opposite sides to form a sling. Lightly coat the pan and parchment with cooking spray or butter.

Melt 8 tablespoons unsalted butter on the stovetop or in the microwave. Pour into a large bowl and let cool for 5 minutes.

Add 1 cup of the packed dark brown sugar to the bowl of butter and whisk until smooth. Add 1 large egg, 2 teaspoons vanilla extract, and ½ teaspoon kosher salt, and whisk to combine. Add 1 ¼ cups all-purpose flour and sprinkle with ½ teaspoon baking powder. Fold together with a flexible spatula until just combined; the batter will be very thick.

Transfer the batter to the pan and spread into an even layer. Place the remaining 2 tablespoons packed dark brown sugar and 2 teaspoons ground cinnamon in the now-empty bowl and stir to combine. Sprinkle evenly over the batter. Using a toothpick or skewer, gently swirl the cinnamon sugar into the blondie batter, running it lengthwise and crosswise.

Bake until the edges pull away slightly from the pan and are firm, and a tester inserted into the center comes out with just a few moist crumbs, 18 to 22 minutes. Place the pan on a wire rack and let cool completely, about 1 hour.

Place ¼ cup powdered sugar, 1 teaspoon milk, and 1 pinch kosher salt in a small bowl and whisk until smooth. Grasp the excess parchment and lift the blondie slab out of the pan and onto a cutting board. Drizzle with the glaze and let sit until the glaze is set, about 15 minutes. Cut into 9 to 12 pieces.